1. Field of the Invention
The present invention relates to a circulator element such as a lumped element isolator or circulator which is used in a high-frequency circuit or the like.
2. Description of the Related Art
Many of cellular phones that are currently used are digital cellular phones. Many of digital cellular phones that are used in Japan employ a PSK (phase modulation) system. In the PSK system digital cellular phone, a linear power amplification circuit is provided at the rear of a transmission circuit, and a transmission antenna is provided at the rear of the power amplification circuit.
The transmission antenna provided in the cellular phone has its impedance significantly changed according to positional relations with hands and a head, and other usage states, and therefore mismatching of impedance occurs between the transmission antenna and the power amplification circuit provided at the front of the transmission antenna. As a result, part of a signal output from the power amplification circuit to the transmission antenna becomes a reflected wave, causing the signal of the power amplification circuit to be distorted. Because the linear power amplification circuit is vulnerable to distortion of the signal, demodulation of the signal becomes difficult if the signal is considerably distorted.
As a measure for avoiding this problem, the conventional cellular phone comprises an isolator between the transmission antenna and the power amplification circuit. The isolator is a circulator element, and a signal input from the power amplification circuit is output to the transmission antenna, but a signal input from the transmission antenna is not output to the power amplification circuit. As a result, distortion of the signal of the power amplification circuit by the reflected wave from the transmission antenna is inhibited.
This type of isolator generally includes at least a magnetic rotator providing a irreversible characteristic, a permanent magnet for applying a direct-current magnetic field to the magnetic rotator, a central conductor placed between the magnetic rotator and the permanent magnet, a capacity substrate for a parallel resonance capacity, a yoke for improving efficiency of the direct-current magnetic field to the magnetic rotator. For the magnetic rotator, YIG (yttrium-iron-garnet) based ferrites, specifically garnet type ferrite materials prepared by adding various kinds of elements to Y3Fe5O12 with Y3Fe5O12 as a basic composition, are usually used. For the permanent magnet for application of a direct-current magnetic field, ferrite magnets are used, and for the capacity substrate, condensers using ceramics for high frequencies having a temperature characteristic of dielectric constant of near 0, glass epoxy resins or other resins developed for high frequencies or the like are used.
The reason why YIG is used for high frequency circuit components such as a circulator and an isolator is that a saturation magnetization (4πMs) suitable for the circuit and a temperature characteristic thereof can be set, and a magnetic resonance half line width (ΔH) representing a magnetic loss and a dielectric loss (tan δ) representing an electric loss are small. Indeed, the magnitudes of the magnetic resonance half line width (ΔH) and the dielectric loss (tan δ) have significant influences on device performance of the circulator and the isolator, and therefore for obtaining a smaller magnetic resonance half line width (ΔH) and dielectric loss (tan δ), studies have been conducted on their compositions, added elements and substituent elements (e.g. Japanese Patent Publication No. 4-74842 (Patent Document 1) and Japanese Patent Laid-Open No. 11-273928 (Patent Document 2)).
[Patent Document 1] Japanese Patent Publication No. 4-74842
[Patent Document 2] Japanese Patent Laid-Open No. 11-273928
However, in the conventional garnet type ferrite material, the range of compositions for a satisfactory magnetic resonance half line width (ΔH) and dielectric loss (tan δ) is so small that the magnetic resonance half line width (ΔH) and the dielectric loss (tan δ) are considerably degraded even with a very small variation in composition, and therefore there is a problem in terms of commercialization, thus making it difficult to realize a circulator element such as an isolator excellent in both insertion loss and temperature characteristic.
There are cases where characteristics as a circulator element such as an isolator cannot be satisfied by merely adjusting characteristics of the garnet type ferrite material. Particularly, even if the temperature characteristic of the garnet type ferrite material itself is improved, the temperature characteristic as the circulator element is not necessarily improved.
Thus, the object of the present invention is to provide a technique for improving a circulator element for its temperature characteristic.